Methanofullerenes such as phenyl-C61-butyric acid methyl ester (PC60BM) and phenyl-C71-butyric acid methyl ester (PC70BM) are widely used as electron acceptor materials in organic photovoltaic devices (OPV) and organic photodetectors (OPD) (G. Dennler et al., Adv. Mater. 2009, 21, 1-16; F. He and L. Yu., J. Phys. Chem. Lett. 2011, 2, 3102-3113; I. Etxebarria et al., Journal of Photonics for Energy 2015, 5, 057214).
The synthesis of methanofullerenes is most commonly based on [3+2] cycloadditions by the reaction of diazomethanes, diazoacetates, diazoamides or diazoketones (A. Hirsch and M. Brettreich, Fullerenes: Chemistry and Reactions, 2005, Wiley-VCH, ISBN: 3-527-60820-2, pp. 119ff). Also, cyclopropanation reactions by nucleophilic addition of deprotonated α-halo-esters or ketones have been used successfully (C. Bingel, Chem. Ber. 1993, 126, 1957-1959). It has been demonstrated that addition of unsymmetrically substituted diazoalkanes leads to two isomers, i.e., [5,6]fulleroids having an open C—C bond on the fullerene cage and [6,6]methanofullerenes (Hummelen et al., J. Org. Chem. 1995, 60, 532-538). While fulleroids are formed first, when submitted to heat or irradiation by light they convert quantitative to the latter, resulting in [6,6]methanofullerenes as the thermodynamically stable compound used for OPV and OPD applications (Hummelen et al., J. Org. Chem. 1995, 60, 532-538) (Kooistra et al., Org. Lett. 2007, 9, 551-554). An additional degree of complexity is reached when C70 instead of C60 is the reactant. Whereas C60 contains only one type of 6-6 bond (a bond between two six membered rings), C70 has four, leading to mixtures of regioisomers. Even larger numbers of different regioisomers are formed in the case of multiple (such as bis- and tris-) adducts. A general discussion of isomers among fullerene adducts in the case of fullerene larger than C60 (such as C70, C76, C78 and C84) can be found in Thilgen et al. (Angew. Chem. Int. Ed. Engl. 1997, 36, 2268-2280.). A more specific description of isomers of phenyl-C71-butyric acid methyl ester (PC70BM) has been provided by Wienk et al. (Angew. Chem. Int. Ed. 2003, 42, 3371-3375, U.S. Pat. No. 7,906,797 B2, U.S. Pat. No. 8,481,996 B2). Using a [3+2] cycloaddition of substituted diazomethane (using tosylhydrazone as reactant), products were analyzed using 1H NMR and 13C NMR. It was concluded that the major isomer (˜85%) is the α-type compound formed by 1,3-dipolar addition to the most “polar” double bond (the C(8)-C(25) bond) yielding a chiral enantiomeric mixture. Two minor isomers (a combined proportion of ˜15%) were also identified as achiral stereoisomeric “type” addends, in which the addend is bound to the C(9)-C(10) double bond (the second most “polar” C═C bond in the C70 skeleton, after the C(8)-C(25) bond). For simplification, in this document, it will not be differentiated between the enantiomers of the α-isomer, which are referred to collectively as the “α-isomer” or “α-PC70BM.” Similarly, the two achiral isomers are not addressed individually, and they are collectively referred to as the “β-isomers” or “β-PC70BM.” The chromatographic separation of these isomers has been found to be extremely challenging and no viable preparative solution exists.
Reactions of semistabilized sulfur ylides generated in situ from corresponding sulfonium salts with C70 have been investigated. Such synthetic approach has been initially reported by Ito et al. (Synlett. 2013, 24, 1988-1992; JP2014-034519A), who prepared dimethyl (5-methoxy-5-oxo-1-phenylpentyl) sulfonium tetrafluoroborate by bromination of methyl 5-phenylpentanoate at the benzyl position, followed by nucleophilic substitution of the bromide with dimethyl sulfide. After optimization of the reaction with C60 resulting in [6,6]-Phenyl-C61-butyric acid methyl ester (PC60BM) without initial formation of the [5,6] isomer, the reaction was carried out with C70, in o-dichlorobenzene (ODCB). [6,6] PC70BM was obtained with 45% isolated yield and the products consisted of the same isomers having a similar component ratio (e.g., α:β 85:15) as in the conventional substituted diazoalkane addition approach by Wienk et al. (Angew. Chem. Int. Ed. 2003, 42, 3371-3375, U.S. Pat. No. 7,906,797 B2, U.S. Pat. No. 8,481,996 B2), e.g., 85:15.
While the reaction of C70 with dimethyl (5-methoxy-5-oxo-1-phenylpentyl) sulfonium salt resulted in an isomeric mixture, reactions of dimethyl-sulfonium ylides with C70 produced methanofullerenes of the type C70CHCOR with regioselectivity (Wang et al., J. Org. Chem. 1996, 61, 5198-5199). In yet other instances, using the methyl-, iso-propyl-substituted sulfonium ylide of methyl 5-phenylpentanoate resulted in enrichment of α-PC70BM to 95% (based on 1H NMR spectroscopy)(JP2014-034519A).
No systematic correlation of the structure of the substituted sulfonium ylide and the extent of regioselectivity has been established.